A DNA-agent has been obtained by phenol extraction of ΦΧ174 bacteriophage. This DNA-agent is infectious for both E. coli C and E. coli B prepared in sucrose-tris buffer and for EDTA-lysozyme spheroplasts of the same cells. It differs from the intact phage in DNase susceptibility, an altered host range and greater temperature stability. Up to the present time the participation of a protein component in the infective principle could be demonstrated neither by inactivation with ΦΧ174 antiserum nor by digestion with proteases.